The majority of present day integrated circuits (ICs) are implemented by using a plurality of interconnected field effect transistors (FETs), also called metal oxide semiconductor field effect transistors (MOSFETs), or simply MOS transistors. An MOS transistor includes a gate electrode as a control electrode and spaced apart source and drain electrodes between which a current can flow. A control voltage applied to the gate electrode controls the flow of current through a channel between the source and drain electrodes.
MOS transistors, in contrast to bipolar transistor, are majority carrier devices. The gain of an MOS transistor, usually defined by the transconductance (gm), is proportional to the mobility of the majority carrier in the transistor channel. The current carrying capability of an MOS transistor is proportional to the mobility of the majority carrier in the channel. The mobility of holes, the majority carrier in a P-channel MOS transistor can be increased by applying a compressive longitudinal stress to the channel. The mobility of electrons, the majority carrier in an N-channel MOS transistor can be increased by applying a tensile transverse stress to the channel. In a silicon MOS transistor such stresses can be applied to the channel of an MOS transistor by appropriately embedding a stress inducing material such as SiGe in the silicon substrate of the transistor. The stresses are caused by lattice mismatches between the SiGe and the host silicon material. The intrinsic stresses in the SiGe redistribute into the adjacent areas of the host substrate, namely into the channel region of the MOS transistor. Unfortunately, one of the problems with embedded SiGe technology is the mechanical stability of the SiGe layers. At elevated temperatures the intrinsic stress in the SiGe layers relaxes due to dislocation generation. The decrease in stress, in turn, causes a reduction in the stress induced mobility increase, and hence a deterioration of device performance.
Accordingly, it is desirable to provide methods for fabricating stressed MOS devices that prevent stress relaxation. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.